Phase Imaging: Beyond Topography


Phase Imaging is a powerful extension of Tapping Mode Atomic Force Microscopy (AFM) that provides nanometer-scale information about surface structure often not revealed by other SPM techniques. By mapping the phase of the cantilever oscillation during the TappingMode scan, phase imaging goes beyond simple topographical mapping to detect variations in composition, adhesion, friction, viscoelasticity, and perhaps other properties. Applications include identification of contaminants, mapping of different components in composite materials, and differentiating regions of high and low surface adhesion or hardness. In many cases, phase imaging complements lateral force microscopy (LFM) and force modulation techniques, often providing additional information more rapidly and with higher resolution. Phase imaging is as fast and easy to use as TappingMode AFM -- with all its benefits for imaging soft, adhesive, easily damaged or loosely bound samples -- and is readily implemented on any MultiMode or Dimension Series SPM with NanoScope III controller equipped with an Extender Electronics Module.

In TappingMode AFM, the cantilever is excited into resonance oscillation with a piezoelectric driver. The oscillation amplitude is used as a feedback signal to measure topographic variations of the sample. In phase imaging, the phase lag of the cantilever oscillation, relative to the signal sent to the cantilever's piezo driver, is simultaneously monitored by the Extender Electronics Module and recorded by the NanoScope III SPM controller. The phase lag is very sensitive to variations in material properties such as adhesion and viscoelasticity.

Once the SPM is engaged in TappingMode, phase imaging is enabled simply by displaying a second image and selecting the Phase data type in the NanoScope software. Both the TappingMode topography and phase images are viewed side-by-side in real time. The resolution of phase imaging is comparable to the full resolution of TappingMode AFM. Phase imaging can also act as a real-time contrast enhancement technique. Because phase imaging highlights edges and is not affected by large-scale height differences, it provides for clearer observation of fine features, such as grain edges, which can be obscured by rough topography.

Phase imaging is a powerful tool for mapping variations in sample properties at very high resolution. It can be turned on while using TappingMode AFM with no cost in speed or resolution, and all NanoScope users are encouraged to add it to their SPM repertoire. Phase imaging can complement force modulation and LFM methods, often with superior image detail, and can in some cases provide information not revealed by these or other SPM techniques. The rapidly growing list of phase imaging applications includes characterizing the components of composite materials, mapping of surface friction and adhesion, and identification of surface contamination. Phase imaging promises to play an important role in the ongoing study of material properties at the nanometer scale.


Author: Hong-Qiang Li: email: hli@uoguelph.ca
Curator: Dan Thomas email: <thomas@chembio.uoguelph.ca>
Last Updated: Thursday, April 24 1997